A minimally invasive procedure is a medical procedure that is performed through the skin or an anatomical opening. In contrast to an open procedure for the same purpose, a minimally invasive procedure will generally be less traumatic to the patient and result in a reduced recovery period.
However, there are numerous challenges that minimally invasive procedures present. For example, minimally invasive procedures are typically more time-consuming than their open procedure analogues due to the challenges of working within a constrained operative pathway. In addition, without direct visual feedback into the operative location, accurately selecting, sizing, placing, and/or applying minimally invasive surgical instruments and/or treatment materials/devices can be difficult.
For example, for many individuals in our aging world population, undiagnosed and/or untreatable bone strength losses have weakened these individuals' bones to a point that even normal daily activities pose a significant threat of fracture. In one common scenario, when the bones of the spine are sufficiently weakened, the compressive forces in the spine can cause fracture and/or deformation of the vertebral bodies. For sufficiently weakened bone, even normal daily activities like walking down steps or carrying groceries can cause a collapse of one or more spinal bones. A fracture of the vertebral body in this manner is typically referred to as a vertebral compression fracture. Other commonly occurring fractures resulting from weakened bones can include hip, wrist, knee and ankle fractures, to name a few.
Fractures such as vertebral compression fractures often result in episodes of pain that are chronic and intense. Aside from the pain caused by the fracture itself, the involvement of the spinal column can result in pinched and/or damaged nerves, causing paralysis, loss of function, and intense pain which radiates throughout the patient's body. Even where nerves are not affected, however, the intense pain associated with all types of fractures is debilitating, resulting in a great deal of stress, impaired mobility and other long-term consequences. For example, progressive spinal fractures can, over time, cause serious deformation of the spine (“kyphosis”), giving an individual a hunched-back appearance, and can also result in significantly reduced lung capacity and increased mortality.
Because patients with these problems are typically older, and often suffer from various other significant health complications, many of these individuals are unable to tolerate invasive surgery. Therefore, in an effort to more effectively and directly treat vertebral compression fractures, minimally invasive techniques such as vertebroplasty and, subsequently, kyphoplasty, have been developed. Vertebroplasty involves the injection of a flowable reinforcing material, usually polymethylmethacrylate (PMMA—commonly known as bone cement), into a fractured, weakened, or diseased vertebral body. Shortly after injection, the liquid filling material hardens or polymerizes, desirably supporting the vertebral body internally, alleviating pain and preventing further collapse of the injected vertebral body.
Because the liquid bone cement naturally follows the path of least resistance within bone, and because the small-diameter needles used to deliver bone cement in vertebroplasty procedure require either high delivery pressures and/or less viscous bone cements, ensuring that the bone cement remains within the already compromised vertebral body is a significant concern in vertebroplasty procedures. Kyphoplasty addresses this issue by first creating a cavity within the vertebral body (e.g., with an inflatable balloon) and then filling that cavity with bone filler material. The cavity provides a natural containment region that minimizes the risk of bone filler material escape from the vertebral body. An additional benefit of kyphoplasty is that the creation of the cavity can also restore the original height of the vertebral body, further enhancing the benefit of the procedure.
Conventional inflatable bone tamps (IBTs) used in kyphoplasty procedures incorporate balloons that exhibit generally symmetrical free expansion profiles about the midlines of the balloons. In other words, the proximal and distal halves of the balloon expand at substantially the same rates and to substantially the same sizes as inflation fluid is delivered to the balloon. Note that “free expansion” refers to expansion in free space, rather than within the target environment, such as within a vertebral body.
In general, a balloon exhibiting symmetrical free expansion will expand along a path of least resistance when inflated within its target environment, which can result in less than optimal procedure results. For example, FIGS. 1A-1C depict the use of a conventional IBT 130 to prepare a fractured vertebra 102 to be filled with bone cement (as part of a kyphoplasty procedure). Due to a compression fracture in vertebral body 102, the spinal column portion represented by vertebral bodies 101, 102, and 103 exhibit an abnormal spinal curvature (kyphosis) CK that can lead to severe pain and further fracturing of adjacent vertebral bodies if left untreated.
As shown in FIG. 1A, kyphoplasty is a performed by creating an access path to the target vertebral body (102) using a cannula 104. An inflatable bone tamp 130 is placed into an interior lumen 104-L of cannula 104. Inflatable bone tamp 130 includes a catheter 132, a balloon 140 at the distal end of catheter 132, and a connector 131 (e.g., a Luer Lock fitting) at the proximal end of catheter 132. Balloon 140 is a conventional kyphoplasty balloon that exhibits a symmetrical free expansion profile.
Inflatable bone tamp 130 is coupled to an inflation syringe 110 by flexible tubing 120. Syringe 110 includes a barrel 111 and a plunger 113 that is slidably disposed in barrel 111. To inflate balloon 140, a force is applied to plunger 113 to express inflation fluid 115 through tubing 120, connector 131, and catheter 132, and into balloon 140. This delivery of inflation fluid 115 cause inflatable structure to begin to expand, as shown in FIG. 1B. As balloon 140 expands, it compresses the surrounding cancellous bone 102-C to create a cavity within fractured vertebra 102. Ideally, balloon 140 would also force endplates 102-E of vertebra 102 apart to restore the vertebral body height lost due to the compression fracture.
However, in many instances, a fractured vertebra “sets” in its fractured condition, as the bone partially heals in its compressed state. Therefore, because, balloon 140 will exhibit expansion along the path of least resistance within vertebral body 102, in such cases balloon 140 will expand towards the posterior of vertebra 102, continuing to compress cancellous bone 102-C rather than forcing apart endplates 102-E, as shown in FIG. 1C. In essence, due to the symmetrical expansion properties of balloon 140, the hard endplates 102-E act as a channel that guides expansion of balloon 140 into the softer cancellous bone 102-C, where the inflation force acts to compress the cancellous bone 102-C, rather than restore the height of vertebra 102. Consequently, the kyphosis (curvature CK) caused by the fracture of vertebra 102 is not corrected.
Accordingly, it is desirable to provide surgical tools and techniques that provide more effective vertebral body height restoration during the treatment of compression fractures.